1. Field of the Invention
The present invention relates to a method of cold rolling a grain-oriented silicon steel sheet having excellent and uniform magnetic characteristics along the rolling direction of a coil and a roll cooling controller for a cold rolling mill using the cold rolling method.
2. Description of the Related Art
Grain-oriented silicon steel sheet, which is mainly used as iron cores for transformers and generators, requires high magnetic flux density (often represented by the magnetic flux density existing in a magnetic field of 800 A/m: B.sub.8) and low iron loss (represented by the iron loss in AC 50 Hz at a maximum magnetic flux density of 1.7 T: W.sub.17/50).
In particular, when a grain-oriented silicon steel sheet is used in large transformers, it must have uniform material characteristics in order to improve its efficiency and noise level. Various efforts have been made to reduce iron loss. The following methods have proven effective:
(1) reducing the thickness of the steel sheet;
(2) increasing its Si content; and
(3) reducing the grain size of secondary recrystallized grains in the final product.
The above improvements can produce a material having an iron loss W.sub.17/50 of 90 W/kg from a product 0.23 mm thick.
It is difficult, however, to further reduce iron loss beyond those levels. Reducing the thickness of a steel sheet to less than 0.23 mm causes defective secondary recrystallization by which the iron loss value is caused to deteriorate. Increasing the silicon content makes cold rolling difficult, and decreasing average grain size to less than 4-8 mm causes defective secondary recrystallization by which the iron loss value deteriorates.
Although there has been improvement of magnetic characteristics along the rolling direction of the coil, no improvement has been achieved with respect to dispersion of magnetic characteristics.
Recently, great improvement in iron loss has been achieved by physically fractionating a magnetic domain by either locally introducing strain on the surface of the steel sheet, or by forming grooves thereon. For example, about 0.10 W/Kg can be eliminated at W.sub.17/50 by introducing local strain on the surface of a steel sheet by applying a plasma jet.
To obtain a material having excellent iron loss properties by the physical fractionation method, it is not necessary to reduce the crystal grain size of the final product beyond that achieved in conventional methods, but it is necessary to reduce sheet thickness and to increase both Si content and magnetic flux density. Since it is difficult or even undesirable to further increase the Si content, the improvement of iron loss entirely depends on how the magnetic flux density of a material having a thin sheet thickness can be improved.
To improve the magnetic flux density of a grain-oriented silicon steel sheet, the orientation of the crystal grains of a product must be largely accumulated in the orientation (110) [001], i.e., in the so-called Goss orientation. The crystal grains in the Goss orientation of the grain-oriented silicon steel sheet can be obtained by secondary recrystallization in final finish annealing.
To cause secondary recrystallization, a so-called selective growth is employed so that only the crystal grains near to the (110) [001] orientation are grown and the growth of the crystal grains in the other orientations is suppressed. An inhibitor must be added to suppress the growth of the crystal grains of the other orientations by forming a dispersed precipitation phase in the steel which serves to suppress the growth of the undesired grains.
Since inhibitors having the strongest suppressing action also have the strongest selective growth effects, many studies have been directed to finding the most effective inhibitors to maximize magnetic flux density. AlN has been found to be the most effective inhibitor. As disclosed in Japanese Patent Publication No. 46-23820, a material having a high magnetic flux density of 1.92-1.95 T at B.sub.10 can be obtained from a steel sheet containing Al in such a manner that the steel sheet annealed prior to final cold rolling is quenched and the rolling reduction in the final cold rolling is controlled to a heavy rolling reduction value of 80-95%.
Further, as disclosed in Japanese Patent Publication No. 63-11406, secondary recrystallization can be stabilized by the addition of Sn and Cu.
The inventions disclosed in above publications, however, improve the averaged characteristics of the steel along the rolling direction of the steel sheet but do not address variations of magnetic characteristics, which variations manifest themselves along the rolling direction of the coil.
We have investigated analytically the effects of cold rolling on the variations of magnetic characteristics as they occur along the rolling direction of a steel sheet.
With respect to rolling technology for grain-oriented silicon steel sheet, Japanese Patent Publication No. 50-37130 discloses a method involving setting the roll diameter used in a final cold rolling to 300 mm.phi. or less.
Japanese Patent Application Laid-Open No. 2-80106 discloses a method for using work rolls having a diameter less than 250 mm.phi. to a first stand in tandem rolling. Further, Japanese Patent Publication No. 54-13846 and Japanese Patent Publication No. 54-29182 disclose effecting an aging treatment between cold rolling paths. Further, Japanese Patent Publication No. 50-26493 and Japanese Patent Publication No. 3-23607 disclose regulating the temperature range of cold rolling.
Although the above disclosures effectively improve average magnetic characteristics to some degree and in some ways, they are not effective in stabilizing the magnetic characteristics along the rolling direction of a coil.
As described above, no effective means exists for producing grain-oriented silicon steel sheet having improved magnetic characteristics while suppressing its fluctuation along the rolling direction of the coil.